Nuclear magnetic resonance spectroscopy has been an important analytical technique since it became available in 1946. When atomic nuclei are placed in a constant, homogenous magnetic field of high intensity and subjected at the same time to a certain selected frequency RF alternating field, a transfer of energy can take place between the radio frequency field and the atomic nuclei to produce what is pictured as a flipping of the nuclei, which nuclei will immediately relax, i.e. flip back where they can reabsorb again resulting in a flipping back and forth. This flipping is called a resonance. More precisely, when a system of nuclei is exposed to radiation of frequency f.sub.o, such that the energy hf.sub.o of a quantum of radiation, where h equals Plank's constant, is exactly equal to the energy difference between two adjacent nuclear energy levels, then energy transitions may occur in which the nuclei may be pictured as flipping back and forth from one allowed orientation to another. The apparatus for such NMR experiments is relatively simple in concept and comprises a large magnet to create a strong fixed field H.sub.o and electronic equipment to generate RF excitation energy (transmitter) which is coupled to an excitation coil. This coil is positioned around and excites a sample being investigated which is usually dissolved by a solvent in a glass test tube. An electronic receiver is also coupled to the coil. The part of the NMR spectrometer in which the sample is placed and where the excitation/receiver coil is mounted is know as the probe. In modern NMR spectrometer equipment, the receiver and transmitter are ordinarily turned on and off very quickly so that the receiver is not receiving when the transmitter is on and vice versa. While the NMR spectrometer is simple in principle, the manufacture and design are very demanding because of the very small signal generated by the processing nuclei.
It is important that the receiver coil be very closely physically coupled to the sample atoms which are dissolved in the solvent and that the receiver coil be filled as much as possible with sample material, i.e., have a large filing factor, because the intensity of the received signal in the receiver coil is related to the number of nuclei that couple to the coil and to the filing factor.
Early in NMR development, it was appreciated that a most important parameter in NMR was the homogeneity of the DC magnetic field, H.sub.o. In fact, when the homogeneity was first improved from the earliest magnets, the ability of the NMR spectrometer to perform high resolution spectroscopy was discovered. High resolution spectra are those spectra where the resonance lines are narrower than the major resonance shifts caused by differences in the chemical environment of the observed nuclei, such as are caused by secondary magnetic fields of the molecules of a sample. Homogeneity is a quality of the DC magnetic field. The goal is perfect homogeneity, meaning that all atoms of the sample coupled to the receiver coil are influenced by a magnetic field having identical direction and magnitude. A large proportion of the total NMR spectrometer improvement effort in recent years has been spent in improving the probe part of the NMR spectrometer instrument to improve the techniques for coupling to the sample and to reduce the noise in the signal.
The noise in the NMR signal has a direct influence on the ability of NMR spectroscopy to provide information about the nuclear structure of the sample. It is known that if the portion of a sample in the test tube which is outside the region known as the interaction region is excited during an experiment that the signal to noise ratio is decreased. The portion of the energy received in the pick-up coil which is derived from the those nuclei outside the interaction region provides a noisier contribution, and the resonance line is said to be "broadened." This effect arises because the fixed magnetic field outside of the interaction region is not as homogeneous as the magnetic field in the interaction region. Accordingly, the sample portions located outside the interaction region will resonate at slightly different frequencies which portions contribute to a broadened line width of the resonance.
Such line broadening makes it difficult to observe relatively weak intensity spectral features nearby such broadened spectral lines. This is particularly a problem in proton experiments in which H.sub.2 O is the solvent. It is known in the prior art to place conductive RF shields around portions of sample which are not in the interaction region to attempt to reduce the excitation of said sample portions by stray RF fields arising from RF feed leads as described in copending U.S. patent application Ser. No. 741,720, filed Aug. 7, 1991, now U.S. Pat. No. 5,192,911 assigned to the same assignee of the instant application. The technique described in that application discloses the addition of conductive cylindrical guard rings around the sample and a conductive disk inside the lower guard ring which disk acts to seal off the lower region by a complete RF shield.
However, to permit air flow around and across the sample for temperature control purposes the sealing disk cannot be used in many cases. Since the space is already very small in these probes, it is difficult to add an additional extension to the axial guard ring when the disk shield seal is not employed.
Accordingly, it is an object of the present invention to reduce the coupling between the lead wires and/or other sources of RF parasitic excitation to the sample outside of the interaction region and to improve the homogeneity of the RF coupling within the interaction region.
It is a further object to permit temperature control functions in a probe while at the same time shielding both top and bottom portions of the sample outside the interaction region.